Spectroscopy of thin nanodiamond layers and membranes

Journal of Non-Crystalline Solids 352 (2006) 1344–1347 www.elsevier.com/locate/jnoncrysol

Spectroscopy of thin nanodiamond layers and membranes R. Kravets a, Z. Remes a, V. Vorlicek b, Z. Bryknar c, M. Nesladek d, J. Potmesil a, A. Poruba a, M. Vanecek a,* a

Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 16253 Prague 6, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague 8, Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Trojanova 13, 12000 Prague 2, Czech Republic d LIST, CEA, Saclay F-91191, France b

c

Available online 15 March 2006

Abstract Optical and photoelectrical spectroscopy was applied to the study of submicron thin nanocrystalline diamond films and membranes on silicon and glass substrates, together with Raman and luminescence spectroscopy. Fourier transform photocurrent spectroscopy was used to study electronic defect states in the nanodiamond films. The deposited films are smooth and fully transparent, with Raman spectra approaching that of polycrystalline diamond and a hydrogen concentration below 0.1%. We observed several peaks in the infrared photocurrent spectrum of nanocrystalline diamond, connected with surface states and defects, in the range 0.25–0.6 eV above the valence band edge.  2006 Elsevier B.V. All rights reserved. PACS: 81.07. b; 81.05.Uw; 73.50.Pz Keywords: Raman scattering; Chemical vapor deposition; Optical spectroscopy; Defects; Nanocrystals; FTIR measurements; Photoconductivity

1. Introduction There is a growing interest in the utilization of thin nanocrystalline diamond (NCD) films and self-supporting membranes as a material for optics, electronics and biosensor applications because of diamond’s superior physical properties and bio-compatibility. Nanocrystalline diamond offers optimal properties because of its smooth surface, excellent optical, mechanical and thermal properties, which can approach single crystal diamond values. These films are cheap to produce and can be deposited over large areas. For a new generation of bio-sensors, such as the Bio-FETs (biologically sensitive field effect transistors) [1], the electronic properties of this material are crucial and the study of its defects and doping is necessary.

*

Corresponding author. E-mail address: [email protected] (M. Vanecek).

0022-3093/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.039

In this paper we report on structural, optical, photoelectrical and electronic properties of submicron thin nanodiamond films deposited on silicon and glass substrates. In some cases, etching was used to create windows in (1 0 0) oriented silicon, leaving behind a clear diamond membrane window. 2. Experimental We have grown, with the help of microwave plasmaenhanced chemical vapor deposition (MWPECVD) thin nanodiamond films (thickness 70–1000 nm, as determined from interference fringes) on silicon (1 0 0) substrates. Polished 1- and 2-in. Si wafers and 15 · 15 mm Si plates were used. Nucleation (bias enhanced nucleation) and deposition details were presented elsewhere [2]. Nanodiamond films were also grown on alkali free glass such as Corning 7059 and Schott AF45 (1 · 1 in. and 1 · 3 in.), after mechanical seeding with diamond grit. Typical deposition conditions were the following: total pressure 30 mbar,

R. Kravets et al. / Journal of Non-Crystalline Solids 352 (2006) 1344–1347

Ohmic graphite, Ti/Au or Al electrical contacts were used for FTPS measurements. Samples with typical electrode configurations are shown in Fig. 1. The samples were placed into an optical liquid nitrogen cryostat and a Fourier transform IR spectrometer equipped with external beam output, current preamplifier and voltage source was used for the FTPS measurements [3,4]. Simple painted colloidal graphite contacts were found satisfactory. 3. Results Typical micro-Raman spectra of nanodiamond (0.3– 0.5 lm thin) grown under similar deposition conditions (except for the deposition temperature which was about 700 C for deposition on glass, compared to our standard 900 C for silicon substrates) are presented in Fig. 2, together with luminescence data measured under the same excitation (Ar laser line 514.5 nm). We observe significant differences in the Raman spectra, even if the average nanograin size is similar for both substrates. For both substrates we observe the ‘nanodiamond feature’ at 1140 cm 1 and the carbon G band around 1550 cm 1. For the silicon sub-

30 (a)

Raman signal intensity (a.u.)

0.5% of methane in hydrogen, Si substrate temperature 900 C. For the glass substrate, the pressure was decreased to 20 mbar to decrease the substrate temperature to approximately 700 C. Typical growth rate was 0.4– 0.2 lm/h. Surface morphology and structural properties were visualized using Scanning electron microscopy, Atomic force microscopy and micro-Raman. Typically, well developed nanograins in the range 20–100 nm have been observed on the silicon substrate [2], compared to about 50 nm nanograins on the glass substrate, with a more even distribution of grain sizes. The high nucleation density ensured complete coalescence of grains before reaching 70– 100 nm layer thickness. Optical properties were measured in a broad spectral range 200 nm to 25 lm by transmittance, reflectance and photothermal deflection spectroscopies. In order to investigate the electronic properties, defects and eventual dopant states in the gap, luminescence and photoelectrical spectroscopies have been used. With the help of Fourier transform photocurrent spectroscopy, FTPS, [3,4] we were able to detect new defect states in the gap, characteristic of nanodiamond and not observed in the polycrystalline CVD diamond thick layers. For future bio-applications, the nanodiamond surface was terminated by amine groups [5]. By directional etching we opened a window 6 · 6 mm in the silicon, leaving flat, perfectly transparent, self-supporting nanodiamond membranes [2], which are very suitable for optical and photoelectrical characterization, without influence from the substrate.

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25

NCD on Si

20 15 NCD on glass

10 5 1000

1200

1400

1600

1800

-1

wavenumber (cm )

35 (b)

photoluminescence (a.u.)

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Fig. 1. Different configurations of samples. (a) and (b) Diamond membrane on a (1 0 0) silicon substrate with etched window (coplanar graphite contacts are drawn at the ‘interface’ and ‘face’ surface correspondingly); (c) diamond film glued to glass (membrane was glued to glass and then torn away from the silicon frame); and (d) diamond film deposited onto the low-alkaline glass, with evaporated interdigitated electrodes.

NCD on glass

25 20 15 10

NCD on Si

5 0

0

1000 2000 3000 4000 5000 6000 7000 -1

wavenumber (cm ) Fig. 2. (a) Raman and (b) luminescence spectra for thin (about 0.4 lm) nanocrystalline diamond (NCD) films deposited on a silicon substrate and low-alkaline glass. The photoluminescence energy scale is relative to the Argon laser 514.5 nm excitation line.

R. Kravets et al. / Journal of Non-Crystalline Solids 352 (2006) 1344–1347

strate, the diamond peak at 1332 cm 1 is well developed, its half-width decreasing as the thickness increases. On the contrary, for the glass substrate only a rudiment around 1340 cm 1 is present (a doublet at 1340/1365) while the 1480 and 1550 cm 1 features are much stronger. The luminescence background (Fig. 2(b)) is stronger for nanodiamond on glass, with a strong peak at 5155 cm 1 relative to the 514.5 nm excitation laser line, in absolute terms this corresponds to an energy of 1.77 eV. Transmittance and reflectance spectra are presented in Fig. 3. The attenuation of transmitted light (and of the interference fringes in general) is due to light scattering, which is more efficient in the blue and UV region of the spectrum, and also because of the glass absorption edge. It does not come from the optical absorption of nanodiamond, which we directly measured by photothermal deflection spectroscopy and which was under the detection limits below the glass absorption edge. From the measurements of transmittance in the region 2800–3000 cm 1 for layers deposited on doubly polished Si wafer, we have estimated the hydrogen content in our films to be equal or less than 0.1 at.% [6]. Layers deposited on silicon and on glass exhibit photoconductivity under broad white light from the FTIR spectrometer, those deposited on Si being 1–2 orders more photosensitive than layers deposited on glass. Therefore, we could apply photoelectric spectroscopy methods, such as FTPS to these films and we also expect reasonably good electronic properties of this material for future electronic applications, e.g. as FETs. Layers with a hydrogenated surface exhibit a hole conductive channel at the surface, with conductivity about one order of magnitude lower than usual for hydrogen terminated polycrystalline CVD diamond [7]. We have applied Fourier transform photocurrent spectroscopy [3,4] in the infrared region to search for electronic defects in this material. The configuration of measured samples is presented in Fig. 1. Reported spectra were taken on samples in configuration (c). The normalized FTPS sig-

nal (i.e. the photocurrent signal from the sample divided by the signal from a spectrally independent detector) is plotted in Figs. 4 and 5. Data were collected in the temperature range 77–300 K, a typical development of signal with temperature can be seen in Fig. 4. The signal around 0.4 eV reaches a maximum for 130 K; as the temperature increases, the peaks in the spectral range 0.25–0.6 eV decrease and finally disappear in noise. In Fig. 5, standard FTPS data taken at 77 K are compared to measurements under additional strong UV illumination from a deuterium lamp and with a measurement

T=77K T=130K T=160K

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FTPS signal (a.u.)

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C

B

0.03 A 0.02 E

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0.4 0.5 0.6 photon energy (eV)

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Fig. 4. Temperature dependence of normalized FTPS signal. Four new peaks labeled ‘A’, ‘B’, ‘C’ and ‘E’ can be seen. The energies of these peaks are 0.36, 0.40, 0.57 and 0.27 eV, respectively, peak A consists of sharp individual peaks. The peaks are not in the regions of water vapor and CO2 absorption.

FTPS signal (a.u.)

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T:320nm NCD/glass R:320nm NCD/glass 1-T-R T:0.75mm Schott AF45 glass

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no bias strong UV bias after 6 days into the cryostat

0.2 0.2

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0.3

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photon energy (eV)

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2 3 photon energy (eV)

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Fig. 3. Transmittance (T) and reflectance (R) spectra of NCD on glass. Transmittance of Schott AF45 low alkali glass is also shown.

Fig. 5. FTPS spectrum of a sample with a hydrogenated surface compared with the spectra of the same sample under external illumination from a focused UV deuterium lamp (strong UV bias), and also after the sample was kept 6 days in vacuum. All spectra were measured at 77 K.

R. Kravets et al. / Journal of Non-Crystalline Solids 352 (2006) 1344–1347

taken while the sample was kept under vacuum for a long time (6 days). For the membranes grown on glass, interdigitated Al contacts were used; these samples are generally less photosensitive to white light than those discussed above, grown on Si substrates. We observed a much lower, noisy signal in the infrared, without detection of any peak structure in the 0.2–0.6 eV region. 4. Discussion Our micro-Raman spectra (Fig. 1) of very thin nanodiamond layers (0.3–0.5 lm thin) are typical for different forms of nanocrystalline diamond and have been already described in the literature [9]. Luminescence excited by the Ar laser (514.5 nm) exhibits a strong background and a line at 1.77 eV (Fig. 2). This has been usually assigned to a Si-carbon vacancy complex or more recently to various impurity-carbon vacancy complexes [10]. We attribute the difference in the nanodiamond properties (Raman, luminescence, photosensitivity) of layers, deposited on silicon and glass, to the lower deposition temperature for the glass substrate, the different nucleation method and the presence of oxygen in the glass. Further study is necessary to elucidate each influence. On the other hand, excellent optical transparency and very low hydrogen content are similar for the layers deposited on both types of substrate. The hydrogen content corresponds to that of good quality CVD polycrystalline material [6,8]. Photocurrent spectra in the IR region, determined by very sensitive Fourier transform photocurrent spectroscopy, were observed for samples grown on silicon substrate and they are difficult to interpret at the present time. The fine structure of peaks in the region A (0.353, 0.359 and 0.367 eV) is persistent under UV bias light and under adsorption of different species (OH, CO, . . .) from air and desorption in vacuum. This is approximately the region of fine structure of the boron acceptor in diamond (0.34–0.37 eV), also observed by FTPS [11]. Boron could originate from slightly B doped Si substrate or from the chamber walls. Detailed inspection reveals differences in both spectra, therefore, we claim that we observe a new bulk defect, characteristic of nanocrystalline diamond. On the other hand, spectral features in the regions E (approximately 0.27 eV), B (0.40 eV) and C (approximately 0.57 eV) are surface dependent (adsorption, desorption) and they have never been observed on fully oxidized nan-

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odiamond layers. Therefore, we tentatively connect them with the surface states, due to adsorbates, on fully or partly hydrogenated diamond surface [7]. 5. Conclusion Submicron thin nanocrystalline diamond films and membranes were deposited by microwave plasmaenhanced CVD on silicon and glass substrates. Raman, luminescence, optical and photoelectrical spectroscopies were applied for characterization. Deposited films are smooth, transparent and self-supporting membranes on silicon frame are flat. The Raman spectrum for the layers deposited on silicon is similar to spectrum of polycrystalline diamond and hydrogen concentration is below 0.1%. Fourier transform photocurrent spectroscopy, applied to the study of electronic transitions in nanodiamond films, observed several peaks in the infrared photocurrent spectrum of nanocrystalline diamond. We tentatively connect these with surface states and bulk defects, about 0.25– 0.55 eV above the valence band edge. Acknowledgements This research was supported by the Projects LC 510, 202/05/2233 and by AV0Z10100521. References [1] W. Yang, R. Hamers, Appl. Phys. Lett. 85 (2004) 3626. [2] V. Mortet, J.D. Haen, J. Potmesil, R. Kravets, I. Drbohlav, V. Vorlicek, J. Rosa, M. Vanecek, Diamond Relat. Mater. 14 (2005) 393. [3] M. Vanecek, A. Poruba, Appl. Phys. Lett. 80 (2002) 719. [4] M. Vanecek, R. Kravets, A. Poruba, J. Rosa, M. Nesladek, S. Koizumi, Diamond Relat. Mater. 12 (2003) 521. [5] Z. Remes, A. Choukourov, J. Stuchlik, J. Potmesil, M. Vanecek, Diamond Relat. Mater., in press, doi:10.1016/j.diamond.2005.10.043. [6] E. Titus, D.S. Misra, A.K. Sikder, P.K. Tyagi, M.K. Simgh, A. Misra, N. Ali, G. Cabral, V.F. Neto, J. Gracio, Diamond Relat. Mater. 14 (2005) 476. [7] J. Ristein, in: C.E. Nebel, J. Ristein (Eds.), Thin-Film Diamond II, Semiconductors and Semimetals, 77, Elsevier, Amsterdam, 2004, p. 37. [8] K.M. McNamara, K.K. Gleason, C.J. Robinson, J. Vac. Sci. Technol., A 10 (1992) 3143. [9] W. Kulish, C. Popov, S. Boycheva, G. Beshkov, V. Vorlicek, P.N. Gibson, G. Georgiev, Thin Solid Films 469&470 (2004) 99. [10] D.V. Musale, S.R. Sainkar, S.T. Kshirsagar, Diamond Relat. Mater. 11 (2002) 75. [11] R. Kravets, M. Vanecek, C. Piccirillo, A. Mainwood, M.E. Newton, Diamond Relat. Mater. 13 (2004) 1785.